Apparatus for fabricating radio frequency attenuators



p 1, 1964 J. M. DREES 3,147,140

APPARATUS FOR FABRICATING RADIO FREQUENCY ATTENUATORS Filed Oct. 51, 1962 2 Sheets-Sheet 1 I 40 ggugg FURNACE COOLANT L/ SUPPLY GAUGES SUPPLY l3 f TRANSDUCER POWER ASSEMBLY SUPPLY FURNACE AND DEPOSITION CHAMBER l 6 HYDQO 3 2 FORMING GAS FORMING czgzs ou CONTROL S fl GAS SUPPLY VALVE VALVE SUPPLY (OHMS/ SQ.)

E INVENTOR.

JOSEPH DREES RESISTIVITY x lo DISTANCE ALONG A, 9 Y

ATTORNEY Sept. 1, 1964 J. M. DREES 3,147,140

APPARATUS FOR FABRICATING RADIO FREQUENCY ATTENUATORS Filed Oct. 31, 1962 2 Sheets-S'neet 2 lE-Z INVENTOR.

JOSEPH M. DREES BY Ab, #f A ATTORNEY United States Patent 3,147,140 APPARATKE FOR FABRICATING RADIO FREQUENCY ATTENUATORS Joseph M. Dress, aratoga, Calif., assignor to Sylvania Electric Products line, a corporation of Delaware Filed Oct. 31, 1362, Ser. No. 234,343 8 Claims. (Cl. 1I847) This invention relates to attenuators for traveling wave tubes and more particularly to deposition ovens which form such attenuators.

A traveling wave tube employs a helix formed of wire to propagate an electromagnetic signal in phase with an adjacent electron beam. The helix is held in proper alignment with the beam by a series of circumferentially spaced support rods contacting the periphery of the helix. In order to enhance coupling between the signal and beam, attenuators of highly conductive material such as carbon are attached to the cylindrical rods in contact with the helix to absorb the backward wave or reflected portion of the signal.

Prior practice of depositing carbon onto the support rods through the cracking of hydrocarbon gases in an oven has produced attenuators that do not operate satis factorily in traveling wave tubes. This is believed to re sult from abrupt changes in attenuator thickness caused by the uncontrolled escape of hydrocarbon gas from the deposition oven as the attenuators are formed. These abrupt changes in thickness, imperceptible by conventional measuring techniques, prevent absorption of the reflected wave which, in turn, causes the forward wave of the signal to lose synchronization with the electron beam. The result is low amplification of the signal or in the extreme, inoperativeness of the tube.

An object of the present invention is the provision of a deposition oven in which precisely uniform and reproducible attenuators for traveling wave tubes are formed.

Another object is the provision of an oven for depositing attenuating material of a precise and controllable thickness on cylindrical rods of small diameters for use in traveling wave tubes or tubes of like construction.

Another object is the provision of such an oven having a cracking chamber constructed to provide a uniform cracking of hydrocarbon gas.

These and other objects of the invention will become apparent from the following description of preferred embodiment thereof, reference being had to the accompanying drawings in which:

FIGURE 1 is a block diagram of a system for depositing carbon onto cylindrical rods;

FIGURE 2 is a side elevation, partially cut-away, of the deposition oven of FIGURE 1 constructed in accordance with the invention;

FIGURE 3 is a side elevation, partially cut-away, of a cracking block assembly of the oven of FIGURE 2 illustrating the cracking zone of the oven;

FIGURE 4 is a transverse section taken along line 4-4 of FIGURE 3; and

FIGURE 5 is a graph showing the relative magnitude of resistivity of an attenuator as a function of distance along the rod.

Briefly, the deposition oven embodying the invention employs standard heating coils toraise the temperature of a cylindrical ceramic rod which is moved in one direction through the oven. At a location near the exit end of the oven, the rod enters a longitudinally extending aperture in a cylindrical cracking block, the aperture being surrounded by inert forming gas having a pressure greater than atmosphere. A cylindrical cracking chamber intersects the axis of the aperture near one end of the block. One end of the chamber opens to the atmosphere and the other end connects to a source of hydrocarbon gas having a pressure greater than atmosphere but less than that of the forming gas. As the rod enters the chamber, the hydrocarbon gas is decomposed and carbon is deposited on the rod. The rod width over which the carbon is deposited at any one instant remains constant during operations because of the pressure balance of the cracking and inert gases within the chamber achieved in part by the relative pressure relationship of these gases. An opening in the cracking block provides for a flow of the forming gas between the central aperture in the block and the oven chamber so as to maintain a constant pressure of the forming gas immediately adjacent to the cracking chamber.

Referring now to FIGURE 1, a deposition system 1 for depositing attenuating carbon is shown schematically and comprises an elongated base 2, a cylindrical deposition oven 3 supported on base 2, a transducer assembly 4 and sources 5 and 6 of hydrocarbon and forming gases. Source 5 of hydrocarbon gas, preferably methane at a pressure P is connected to oven 3 through valve 7. Source 6 of inert forming gas, preferably parts nitrogen and 5 parts hydrogen at pressure P is connected to the oven through valve 8. The oven 3 is surrounded by air at a pressure P Oven 3 preferably is an electric oven and is energized by power supply 9. Temperature gauges 1i) record the mean furnace temperature. Opposite ends of the furnace connect to a coolant supply 12. These ends also have apertures to receive a ceramic rod 13 which is driven through the oven along a horizontal axis A by transducer assembly 4 connected to power supply 14. Attenuating material is deposited on the rod as it passes through the oven.

Referring now to FIGURE 2, deposition oven 3 constructed in accordance with the invention is supported above base 2 by end plates 21 and by side plates, one of which is indicated at 21. The oven has a cylindrical heater shell 22 with an inner ceramic wall 22a, a gas cracking block 23 within wall 22a, and elongated plug members 24 and 25 in opposite ends of wall 22a and having axes coincident with axis A. Plugs 24 and 25 and inner wall 22a define a cylindrical central chamber 27 within which cracking block 23 is located.

The source of heat for the oven is heater coils 28 located within heater shell 22, the coils being connected to a power supply via leads 29. The heater shell is formed of a laminated series of concentric cylinders arranged about the heater coils and includes, in addition to ceramic wall 22a, an intermediate heater support sleeve 39, outer insulator 31 and outer metallic covering 32. The cylinders terminate at end walls 33 and 34, with the exception of the ceramic wall 22a which extends through apertures in these walls to firmly engage plug members 24 and 25.

Heat is confined to the central region of the oven by outer insulator 3i and by first and second planar plates 35 attached by three circumferentially spaced bolts 36 to the end Walls of the shell. Thermocouples are located within the oven and connect to gauges 10, FIGURE 1, through leads 37.

Each plug 24 and 25 comprises a cylindrical metallic block, preferably made of copper, formed with a reduced forward portion 40 attached to support plate 20, an enlarged central portion 41, and a reduced rearward portion 42 which fits into heater shell Wall 22a. Cooling coils 43 are wound about the central plug portion 41 and connect by conduits 44 to refrigerator 12, FIGURE 1. These coils cool the ends of the oven which serves to cool the rod after the depositing process. Radiating fins 45 on the inner ends of the plugs further help to confine the heat to the interior oven parts.

Each plug 24 and 25 also has a central aperture 46 supporting an eiongated bushing 47 through which each rod is guided in its travel through the oven. Bushing 47 preferably is made of a material having a low coefficient of friction and heat conductivity, and is longer than the apertures 46 so as overhang the end of the plug as shown.

Also formed within each of the plugs 24 and 25 parallel with axis A are cylindrical passageways 48 and Forming gas passes through passageways 48 from source 6, FIGURE 1, via flexible hoses 50 to the central chamber 27. Passageways 49 in plugs 24 and 25 support conduits 51 and 52, respectively, which extend into chamber 27 for connection with cracking block 23. Conduit 51 opens to the atmosphere surrounding the oven and so exhausts the cracking block to atmosphere. Conduit 52 connects the block with source 5, FIGURE 1, of hydrocarbon gas via fiexible hose 53.

As shown in FIGURE 3, a cracking block 23 comprises a cylindrical block of highly conductive heat resistant material, such as molybdenum, and has a central aperture 60 through which the rod 13 passes, a side wall 61 and end surfaces 62 and 63. The diameter of central aperture 60 over a major portion of its length is greater than the rod diameter to permit the flow of forming gas into the aperture and surround a substantial length of the rod within the block. The diameter of the aperture over the remainder of its length is reduced at shoulder 64 to a size slightly greater than that of the rod to block or impede axial flow of gases along the rod without subjecting the rod to substantial frictional drag as it moves axially. A radially extending breathing hole 65 is formed in the side wall in order to accommodate the passage of forming gas through the enlarged portion of the aperture to the exterior of the block so as to maintain constant forming gas pressure in the aperture even when variations in the rate of rod feed occur.

The cracking block is also formed with a Z-shaped gas flow channelway comprising an input leg 66, a transverse cracking chamber 67, and an output leg 68. Input leg 66 extends almost the full length of the block parallel with axis A and is connected at end surface 63 to conduit 52 and to chamber 67 at junction 69. Transverse chamber 67 has an axis which is normal to axis A and intersects the central aperture 60 of the block at 79. Chamber 67 preferably is cylindrically shaped having a diameter greater than the diameter of the reduced portion of aperture 60, and is located equidistant between the end surface 62 and shoulder 64 for reasons explained hereinafter. The hydrocarbon gas undergoes cracking Within this leg 67 which is, therefore, the cracking chamber of the cracking block.

The output leg 68 of the channelway is parallel with axis A and joins the cracking chamber 67 at junction 72. Leg 68 also is cylindrically shaped and receives the end portion of exhaust conduit 51 which connects the cracking chamber to atmosphere.

In operation, heater coils are energized to bring oven 3 up to heat and the cooling system is turned on. Hydrocarbon gas is then admitted through line 53, see FIGURE 2, to preheated cracking block 23. The pressure P of this gas is greater than atmospheric pressure P exterior of the oven. Simultaneously, forming gas at pressure P greater than the pressure P of the atmosphere enters central chamber 27 of the oven from conduits t and through plug passageway 43. The forming gas flows from chamber 27 into the cracking block through the breathing hole 65, see FIGURE 3, and through the enlarged portion of the central aperture 69. An important feature of the oven is that the forming gas does not mix with the hydrocarbon gas within the adjacent cracking chamber 67 because of the pressure balance at the point of intersection of central aperture 60 and the cracking chamber 67, i.e., planes BB, and CC, FIGURE 3.

Rod 13 is connected at one end to transducer assembly 4, see FIGURE 1, which is preferably a cam-follower and travels through bushing 46 in plug and into the cracking block. The drive level of the transducer is ad 4- justed so that the rod travels through the oven at a varying but predetermined velocity and is synchronized with the flow rate of the hydrocarbon gas to enter the cracking block as cracking of the latter begins, the amount of deposition depending on the time the rod is within chamber 67.

As rod 13 leaves the cracking chamber, forming gas surrounding the block prevents oxygen of the atmosphere from recombining with the hot carbon deposit. The temperature at the ends of the oven is then quickly brought below the recombination temperature of the carbon by the coolant circulating around plugs 24 and 25. After passing from the oven, the rod undergoes further processing and is assembled with similarly processed rods into a group for incorporation within a traveling-wave tube or similar electronic discharge device.

The gas pressure balance at the cracking chamber 67 as the hydrocarbon gas decomposes is an important aspect of the invention. Such a balance is maintained by forming the reduced diameter portion of the central aperture 6% of the cracking block 23, see FIGURE 3, symmetrically with respect to the axis of the cracking chamber 67. In other words, the midpoint of chamber 67 between planes C and B is at the midpoint of the distance D between end surface 62 of the cracking block and shoulder 64 of the central aperture. Forming gas is thus symmetrically located about the chamber and confines the hydrocarbon gas within chamber 67. The pressure balance at planes C and B means carbon is only deposited on the portion of the rod periphery which, at any instant, lies between these planes.

Deposition of carbon within this precisely defined area is further facilitated by exhausting of the cracking chamber 67 to atmosphere. Residual gases of decomposition thus pass out of the chamber and do not adversely affect the gas pressure relationship mentioned above.

The oven embodying this invention has provided highly controlled and hence reproducible attenuation patterns. By way of example, the attenuation characteristic for a ceramic rod having a diameter of .100 inch is illustrated in FIGURE 5. The abscissa is in terms of electrical resistivity in ohms per square inch; i.e., the resistance measured between edges of a resistor having its edge distance equal the length between such edges, and the ordinate is in terms of inches measured along the rod. In the plot, attenuation curve 74 comprises an acclivity portion which begins at point E and rises smoothly to point P and a declivity portion which connects points F and G. The horizontal line 75 represents the maximum resistivity of the attenuator and is 10.90 ohms/square. The distance H defines the ends of attenuator pattern which is reproducible to within :040 inch.

This invention is not limited to particular details of construction materials or processes described above as many equivalents will suggest themselves to those skilled in the art. It is the intention, therefore, that the invention be limited only by the appended claims.

\Vhat is claimed is:

1. An oven for depositing attenuating material on an elongated rod having a given diameter, said oven having a central chamber,

a source of inert gas connected to said chamber,

a cracking block having an axis and disposed within said chamber, said block having an axially extending aperture through which said rod passes, part of the length of said aperture having a diameter greater than said rod diameter and the remainder of the aperture length having a diameter substantially equal to said rod diameter,

said block having a cracking chamber intersecting said aperture along said remainder of its length,

a source of cracking gas connected to one end of the cracking chamber, and

means for exhausting the other end of said cracking chamber externally of said oven.

2. The oven according to claim 1 in which the pressure of the inert gas is greater than the pressure in the cracking chamber, and the latter pressure is greater than the pressure externally of the oven.

3. The oven according to claim 2 in which the inert gas comprises 95 parts nitrogen and 5 parts hydrogen and wherein said cracking gas is substantially gaseous hydrocarbons.

4. The oven according to claim 1 in which said block has an opening connecting the central chamber of the oven with said part of the aperture length adjacent to said remainder length.

5. The oven according to claim 1 in which the cracking chamber of the block intersects the aperture midway along said remainder length.

6. The oven according to claim 5 wherein said cracking chamber intersects the aperture at an angle of substantially 90 degrees.

7. An oven for depositing attenuating material on an elongated rod having a given diameter, said oven having an inner surface defining a central chamber with an axis, and having means for heating said chamber,

a source of forming gas connected to said chamber,

a cracking block disposed coaxially within said chamber and spaced from the inner surface thereof, said block having an aperture extending axially through 6 the full length of the block and through which said rod passes, the entire length of said aperture having a first part with a diameter greater than said rod diameter and a second part shorter than said first part with a diameter substantially equal to said rod diameter,

said block having a cracking chamber formed therein and intersecting said second part of the aperture,

a source of hydrocarbon gas connected to one end of the cracking chamber,

means for exhausting the other end of said cracking chamber externally of said oven, and

the pressure of said forming gas being greater than the pressure within said cracking chamber whereby the hydrocarbon gas is confined exclusively to the cracking chamber.

8. The oven according to claim 7 with means for circulating forming gas within said first part of said aperture comprising an opening in said block connecting the central chamber of the oven With the first aperture part adjacent to the second aperture part.

References Cited in the file of this patent UNITED STATES PATENTS Re. 24,231 Matejka Oct. 30, 1956 

1. AN OVEN FOR DEPOSITING ATTENUATING MATERIAL ON AN ELONGATED ROD HAVING A GIVEN DIAMETER, SAID OVEN HAVING A CENTRAL CHAMBER, A SOURCE OF INERT GAS CONECTED TO SAID CHAMBER, A CRACKING BLOCK HAVING AN AXIS AND DISPOSED WITHIN SAID CHAMBER, SAID BLOCK HAVING AN AXIALLY EXTENDING APERTURE THROUGH WHICH SAID ROD PASSES, PART OF THE LENGTH OF SAID APERTURE HAVING A DIAMETER GREATER THAN SAID ROD DIAMETER AND THE REMAINDER OF THE APERTURE LENGTH HAVING A DIAMETER SUBSTANTIALLY EQUAL TO SAID ROD DIAMETER, SAID BLOCK HAVING A CRACKING CHAMBER INTERSECTING SAID APERTURE ALONG SAID REMAINDER OF ITS LENGTH, A SOURCE OF CRACKING GAS CONNECTED TO ONE END OF THE CRACKING CHAMBER, AND MEANS FOR EXHAUSTING THE OTHER END OF SAID CRACKING CHAMBER EXTERNALLY OF SAID OVEN. 